1. Field of the Invention
The present invention relates to a fixation device for a spinal column. In particular, the present invention relates to a fastener for connecting a member which extends between vertebrae to one of the vertebrae.
2. Description of the Prior Art
Surgically implantable fixation devices for connecting vertebrae of a deformed or degenerated spinal column are known. U.S. Pat. No. 4,648,388 discloses one such fixation device for interconnecting at least a pair of vertebrae of a spinal column. The fixation device includes an elongate rod connected with the vertebrae. A screw is threaded into a vertebra and has a portion extending from the vertebra. A clamp is placed on the portion of the screw extending from the vertebra. The clamp has a contractable opening for receiving a portion of the rod. A nut is threaded onto the screw against the clamp to contract the opening of the clamp around the portion of the rod. Friction and/or a mechanical interlock between the clamp and the portion of the rod prevents movement of the rod relative to the clamp and screw and, thus, the vertebra.
German Patent No. 26 49 042 discloses a screw having a threaded portion for connection with a vertebra The screw has an intermediate portion fixed to the threaded portion and a pair of legs extending from the intermediate portion. The intermediate portion and pair of legs define a cavity in the screw for receiving a portion of a threaded rod. A pair of nuts are threaded onto the rod and engage the screw from axially opposite directions. The nuts are tightened against the screws to block movement of the rod relative to the screw.